http://www.geo.mtu.edu/EHaz/ConvergentPlatesClass/macpherson/martin_etal_2005.pdf

An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution

Orogenic gold deposits : A proposed classification in the context of their crustal distribution and relationship to other gold deposit types

Orogenic gold and geologic time: a global synthesis

The continental crust has an andesitic composition with high Mg/(Mg+Fe) and Ni contents which may be too high to have formed by differentiation of basaltic magmas. Instead, mantle-derived, high Mg# andesites (HMA) may form a substantial component of the crust. HMA may be produced by partial melting of previously depleted, subsequently metasomatised mantle peridotite. However, they are more likely produced by reaction between ascending melts and mantle peridotite. HMA are less common than basalts among lavas in modern island arcs, but may have been more common in the past, may be produced in specific environments (such as “ridge subduction”), may be more common among plutonic rocks in the lower and middle crust than among lavas at the surface, and may be selectively preserved during later erosion and subduction processes.

Genesis of high Mg# andesites and the continental crust

The continental crust has an andesitic compo- sition with high Mg/(Mg+Fe) and Ni contents which may be too high to have formed by differentiation of basaltic magmas. Instead, mantle-derived, high Mg#e an- desites (HMA) may form a substantial component of the crust. HMA may be produced by partial melting of previ- ously depleted, subsequently metasomatised mantle pe- ridotite. However, they are more likely produced by reac- tion between ascending melts and mantle peridotite. HMA are less common than basalts among lavas in mod- ern island arcs, but may have been more common in the past, may be produced in specific environments (such as "ridge subduction"), may be more common among plu- tonic rocks in the lower and middle crust than among lavas at the surface, and may be selectively preserved during later erosion and subduction processes.

http://rd.springer.com/content/pdf/10.1007%2FBF00311004.pdf

Genesis of high andesites and the continental crust

The Kolar Schist Belt of the Dharwar Craton of South India is an Archean greenstone belt dominated by 

metavolcanic rocks. The mafic metavolcanic rocks occur as komatiitic and tholeiitic amphibolites. The komatiitic 

amphibolites occur along the margins of the N-S trending, synformal belt. They are much less abundant than the 

tholeiitic amphibolites and have 14 to 21-3 wt. per cent MgO. The komatiitic amphibolites from the west/central part 

of the belt have two distinctive REE patterns: (1) those enriched in the middle to light REE but depleted in Ce 

relative to Nd; and (2) those with patterns that are convex up, i.e. depleted in both light and heavy REE, although 

more depleted in the light REE. Associated tholeiites have light REE depleted to flat REE patterns. Komatiitic and 

tholeiitic amphibolites from the eastern part of the belt have enriched light REE patterns. The tholeiitic 

amphibolites from the Kolar Schist Belt are similar to the TH I and TH II types of Archean tholeiites of Condie 

(1981). The komatiitic amphibolites are similar to komatiites and komatiitic basalts of Barberton Mountainland, but 

have higher FeO and TiO 2 abundances and lower Yb/Gd ratios. The petrogenetic interpretations for these 

rocks are based primarily on a modification of the MgO-FeO diagram of Hanson & Langmuir (1978), and modelling of 

Zr, Ni and REE. All of the rocks have undergone some fractionation. While the modelling does not give accurate 

temperatures, pressures, compositions and extents of melting of the mantle sources for the various amphibolites, it 

does present an approach which can be used for estimating these parameters. For example, the komatiitic amphibolites 

appear to be derived from melts generated by 10 to 25 per cent melting of the mantle over a range of depths and 

temperatures greater than 80 km and 1575°C. The variation in the P-T conditions of magma generation is possibly 

due to adiabatic melting in mantle diapirs with a range of FeO/MgO ratios. If the tholeiitic amphibolites are 

derived from similar mantle sources (it is not clear that they are), their parent melts may have been generated by 

similar extents of melting, but at depths of less than 80 km. The komatiitic amphibolites from the west central part 

of the belt were generated from light REE depleted mantle, whereas those from the eastern part of the belt appear to 

have been generated from light REE enriched mantle. The sources for the komatiitic amphibolites in both areas were 

significantly enriched in FeO relative to pyrolite. Thus, a light REE depleted and a light REE enriched source 

appear to have provided mafic volcanics with similar major element chemistry to this belt during its 

evolution.

Geochemistry and Petrogenesis of Amphibolites, Kolar Schist Belt, South India: Evidence for Komatiitic Magma Derived by Low Percentages of Melting of the Mantle

Orthogneisses adjacent to the Kolar Schist Belt were emplaced as tonalitic to granodioritic magmas between 2630 and 2530 Ma. An orthogneiss unit east of the Kolar Schist Belt apparently was a homogeneous granodioritic intrusion at about 2532 Ma, cooling to the blocking temperature of the U-Pb system in sphene by 2521-2517 Ma. Gneisses on the west side of the Kolar Schist Belt were emplaced as at least three separate intrusions dated at 2631, 2610, and 2552 Ma. Evidence from an older basement into which these rocks were intruded comes from inherited zircons with an age of >2800 Ma. Rocks with zircons of the appropriate age to be part of this basement occur as tectonic fragments and inclusions in the area. Gneisses west of the Kolar Schist Belt were last cooled through the blocking temperature of the U-Pb system in sphene at about 2552 Ma, synchronous with the last major intrusive event. Late felsic dikes and plutons apparently postdate deformation and yield sphene Pb-Pb ages of <2480 Ma. A sample of muscov...

U-Pb Ages of Zircon and Sphene for Two Gneiss Terranes Adjacent to the Kolar Schist Belt, South India: Evidence for Separate Crustal Evolution Histories

The late Archean, north-south trending Kolar Schist Belt in south India, 4 km wide by 80 km long, is thought to be a suture between two gneiss terranes (Krogstad et al. 1989). Within this volcanics-dominated belt are recognized both tholeiitic and high Mg (komatiitic and picritic) amphibolites, which make up some 70% and 5% respectively of the exposed outcrops. A massive tholeiitic amphibolite separates the belt into western and eastern parts. A Pb-Pb whole-rock age of 2732±155 Ma on samples from a single outcrop of massive tholeiite is a minimum age for this rock. Samples of this rock have ɛNd values at 2700 Ma that range between +3.8 and +6.8, μ1 (initial 238U/204Pb) of 7.5 and K1 (initial 232Th/238U) of about 4. Two different types of high-Mg amphibolites are recognized from the western part of the belt: a picritic or P-type, and a komatiitic or K-type. The P-type have highly variable Ce/Al ratios all greater than chondritic, Nd/Yb ratios greater than chondritic, ɛNd at 2700 Ma of +1.5 to +8, and Pb isotope compositions variable in 207Pb/204Pb with μ1 of about 8.0 and k1 of about 4. The trace-element data suggest that the light-REE enrichment is a character of the mantle source and is not due to residual garnet. The K-type amphibolites have near chondritic Ce/Al and Nd/Yb ratios, ɛNd at 2700 Ma of +1.5 to +8, and μ1 of about 8 and k1 of about 4. Although the P-type is light-REE enriched compared to the K-type, both types have similar Ce/Nd ratios as well as initial Pb and Nd isotopes. If the 2696±136 Ma age for the Sm-Nd isochron which includes both types of high-Mg amphibolite has any significance it dates the time of light-REE enrichment of the mantle source for the P-type komatiitic amphibolites. The high-Mg amphibolites in the eastern part of the belt are light-REE enriched, have Pb isotopic compositions that are variable in 207Pb/204Pb with a μ1 about 8.5 and ɛNd at 2700 Ma of +1.8 to +4.5. Hydrothermal fluids associated with metamorphism and shearing prior to about 2400 Ma ago were responsible for the introduction of gold-quartz-carbonate veins into the Kolar Schist Belt. The Pb isotope composition of galena in these veins suggests that these fluids may have also introduced extraneous Pb from adjacent older granitoid gneisses into the amphibolites, which could be responsible for the variability in the 207Pb/204Pb ratios of the samples. This extraneous Pb probably is not responsible for the distinct Pb isotope character of each type of amphibolite.

Pb and Nd isotope constraints on the origin of high Mg and tholeiitic amphibolites, Kolar Schist Belt, South India

The Archean Kolar Schist Belt, south India, is a suture zone where two gneiss terranes and at least two amphibolite terranes with distinct histories were accrted. Amphibolites from the eastern and western sides of the schist belt have distinct incompatible element and isotopic characteristics sugesting that their volcanic protoliths were derived from dint mantle sources. The amphibolite and gneiss terranes were juxtaposed by horizontal compression and shearing between 2530 and 2420 million years ago (Ma) along a zone marked by the Kolar Schist Belt. This history of accretion of discrete crustal terranes resembles those of Phanerozoic convergent margins and thus suggests that plate tectonics operated on Earth by 2500 Ma.

http://science.sciencemag.org/content/sci/243/4896/1337.full.pdf

Plate tectonics 2.5 billion years ago: evidence at kolar, South India.

Conspicuous Nd, Sr and Pb isotopic differences exist between the Archean gneiss terranes adjoining the suture at the Kolar Schist Belt, south India. These gneisses, which are the deformed equivalents of plutonic and volcanic rocks, have known or inferred igneous ages of 2630 to 2530 Ma. Initial isotopic ratios of Nd, Sr and Pb suggest that metaplutonic gneisses west of the Kolar Schist Belt were emplaced into, and variably contaminated by, an evolved continental crust that formed prior to 3200 Ma. Felsic metaigneous gneisses that occur as slivers on the western margin of the schist belt have an isotopic character similar to that of the metaplutonic rocks on the same side of the Kolar Schist Belt. On the east side of the Kolar Schist Belt the isotopic evidence suggests that the 2530 Ma granitic gneisses were not derived from or contaminated by an older continental crust. Their source probably evolved with a Nd isotopic composition similar to that of typical Archean mantle, but became light rare earth element enriched after 2900 to 2700 Ma. The inferred tectonic setting for the west side of the Kolar Schist Belt is an Andean continental magmatic arc. For the east side of the Kolar Schist Belt, a possible Phanerozoic analog is an evolved island arc, such as Japan.

Sources of continental magmatism adjacent to the late Archean Kolar Suture Zone, South India: distinct isotopic and elemental signatures of two late Archean magmatic series

V. Rajamani

Papers

Geochemistry and Petrogenesis of Amphibolites, Kolar Schist Belt, South India: Evidence for Komatiitic Magma Derived by Low Percentages of Melting of the Mantle

U-Pb Ages of Zircon and Sphene for Two Gneiss Terranes Adjacent to the Kolar Schist Belt, South India: Evidence for Separate Crustal Evolution Histories

Pb and Nd isotope constraints on the origin of high Mg and tholeiitic amphibolites, Kolar Schist Belt, South India

Plate tectonics 2.5 billion years ago: evidence at kolar, South India.

Sources of continental magmatism adjacent to the late Archean Kolar Suture Zone, South India: distinct isotopic and elemental signatures of two late Archean magmatic series